Cyclin‐dependent kinase 9–cyclin K functions in the replication stress response

Cyclin‐dependent kinase 9 (CDK9) is a well‐characterized subunit of the positive transcription elongation factor b complex in which it regulates transcription elongation in cooperation with cyclin T. However, CDK9 also forms a complex with cyclin K, the function of which is less clear. Using a synthetic lethal RNA interference screen in human cells, we identified CDK9 as a component of the replication stress response. Loss of CDK9 activity causes an increase in spontaneous levels of DNA damage signalling in replicating cells and a decreased ability to recover from a transient replication arrest. This activity is restricted to CDK9–cyclin K complexes and is independent of CDK9–cyclin T complex. CDK9 accumulates on chromatin in response to replication stress and limits the amount of single‐stranded DNA in cells under stress. Furthermore, we show that CDK9 and cyclin K interact with ataxia telangiectasia and Rad3‐related protein and other checkpoint signalling proteins. These results reveal an unexpectedly direct role for CDK9–cyclin K in checkpoint pathways that maintain genome integrity in response to replication stress.